Intermolecular CH⋯O hydrogen bonds in formyl-substituted diphenylhexatriene, a [2+2] photoreactive organic solid: Crystal structure and IR, NMR spectroscopic evidence

Experimental and theoretical insights into the formation of weak hydrogen bonds and H⋯H bonding interactions in the solid-state structure of two eucalyptol derivatives

We report the synthesis and X-ray solid-state structure of two eucalyptol derivatives. Both compounds form self-assembled dimers establishing C–H⋯O hydrogen bonds and H⋯H bonding interactions.

First steps in growth of a polypeptide toward β-sheet structure

The full conformational energy surface is examined for a molecule in which a dipeptide is attached to the same spacer group as another peptide chain, so as to model the seminal steps of β-sheet formation. This surface is compared with the geometrical preferences of the isolated dipeptide to extract the perturbations induced by interactions with the second peptide strand. These interpeptide interactions remove any tendency of the dipeptide to form a C5 ring structure, one of its two normally stable geometries. A C7 structure, the preferred conformation of the isolated dipeptide, remains as the global minimum in the full molecule. However, the stability of this structure is highly dependent upon interpeptide H-bonds with the second chain. The latter forces include not only the usual NH···O interaction, but also a pair of CH···O H-bonds. The secondary minimum is also of C7 type and likewise depends in part upon CH···O H-bonds for its stability. The latter interactions also play a part in the tertiary minimum. A two-strand β-sheet structure is not yet in evidence for this small model system, requiring additional peptide units to be added to each chain.

Fluorescence properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatriene (n = 1, 2): effects of internal rotation

The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λ(a)) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λ(f)) (1, 545 nm; 2, 453 nm) and quantum yields (φ(f)) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λ(ex)), whereas the spectrum of 2 was weakly λ(ex)-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λ(a) = 437-438 nm, λ(f) = 496-505 nm, φ(f) = 0.04-0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck-Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)-CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar-CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.

CAF@ZIF-8: one-step encapsulation of caffeine in MOF

Two strategies for encapsulating caffeine in ZIF-8 were carried out in this work: (1) one-step, in situ encapsulation where caffeine is added to a ZIF-8 synthesis solution and the MOF structure is formed around the entrapped molecule; and (2) ex situ encapsulation whereby caffeine is put into contact with previously synthesized or purchased ZIF-8. The products obtained were analyzed with XRD, TGA, Vis-UV, GC-MS, FTIR, (13)C NMR, and N 1s XPS to compare both encapsulation methods. Chemical and structural evidence indicated that the preferential adsorption site of caffeine molecules inside the ZIF-8 structure is near the methyl and CH groups of 2-methylimidazole ligand. These two groups interact with caffeine by van der Waals forces with methyl groups and via CH···O hydrogen bonds with C═O groups, respectively. In addition, the one-step encapsulation of caffeine in ZIF-8 produced high guest loading (ca. 28 wt % in only 2 h at 25 °C) and controlled release (during 27 days).